This research proposal has two main objectives in the study of relaxin. (1) The establishment and characterization of specific cell receptors for relaxin in target tissues of the rat. Two different approaches will be taken in this study. The first will involve binding studies of (125I)-labeled relaxin to tissue homogenates of target tissues (uterus, cervix, mammary gland) and control tissues (adrenal and ileum). Once tissue and hormone specificity are determined, then other binding studies will be conducted to characterize the receptor. One criticism of the above procedure is that tissue homogenates contain a heterogeneous population of cells. Therefore, the cell type which binds the hormone is questionable. As a result, a second approach in the investigation of binding sites for relaxin will be to conduct autoradiographic studies on section of target tissues using labeled relaxin to clarify which cells are binding the hormone. (2)Determination of the hormonal factor(s) which influence the synthesis of relaxin in the corpus luteum of pregnancy in the rat. The organ culture of corpora lutea of pregnancy in the latter half of gestation under varying hormonal conditions will be the methodology employed to determine which hormones influence relaxin synthesis. Results will be determined by bioassay of aqueous tissue extracts and by the morphology of luteal cells.